In known air classification systems unclassified mixed materials may be deposited by a suitable feed conveyor into the lower inlet end of an inclined rotating air classifier drum. As the drum rotates, a stream of air is forcefully directed through the drum and entrains light weight, light density materials which comprise part of the mixture and carries them out the raised discharge end of the drum into a suitable storage area such as a plenum, cyclone or silo. The materials which are too heavy or too dense to be entrained are tumbled by the rotation of the drum and eventually work their way back out the lower inlet end of the drum onto suitable conveyor means which removes them for subsequent processing or storage.
Such air classification systems are becoming popular in municipal waste handling installations where a great variety of materials are mixed together. For example, paper fabric, plastic, glass and wood are mixed with metal which may comprise small items such as coins to large items such as iron castings, for examples. Sometimes the mixture is predominantly comprised of the heavy items and at other times the light items predominate.
When heavy items are predominant, they tend to clog the drum unless the feed rate is reduced since heavy items must work their way gradually down the inclined drum. When light items predominate, separation is difficult unless the velocity of the air stream passing through the drum is increased.
One known attempt to solve this problem has been disclosed in U.S. Pat. No. 3,970,547 which teaches a system wherein the feed conveyor is movable axially within the drum to adjust the longitudinal position within the drum at which the materials are deposited. However, while this has provided some improvement, it has not been entirely successful.